1. Field of the Invention
The mammalian immunological system is unique in its broad ability to produce protein compounds having extremely high specificity for a particular molecular structure. That is, the proteins or immunoglobulins which are produced have a conformation which is specifically able to complement a particular structure, so that binding occurs with high affinity. In this manner, the mammalian immune system is able to respond to invasions of foreign molecules, particularly proteins in surface membranes of microorganisms, and toxins, resulting in detoxification or destruction of the invader, without adverse effects on the host.
The primary immunoglobulin involved in the defensive mechanism is gamma-globulin (IgG). This immunoglobulin, which is a glycoprotein of about 150,000 daltons, has four chains, two heavy chains and two light chains. Each of the chains has a variable region and a constant region. The variable regions are concerned with the binding specificity of the immunoglobulin, while the constant regions have a number of other functions which do not directly relate to the antibody affinity.
In many situations it would be desirable to have molecules which are substantially smaller than the immunoglobulins, while still providing the specificity and affinity which the immunoglobulins afford. Smaller molecules can provide for shorter residence times in a mammalian host. In addition, where the immunoglobulin has to be bound to another molecule, it will be frequently desirable to minimize the size of the final product. Also there are many economies in being able to produce a smaller molecule which fulfills the function of a larger molecule.
There are situations where it will be desirable to be able to have a large number of molecules compactly held together. By having smaller molecules, a greater number can be brought together into a smaller space. Furthermore, where the binding molecule can be prepared by hybrid DNA technology, one has the opportunity to bind the binding portion of the molecule to a wide variety of other polypeptides, so that one can have the binding molecule covalently bonded at one or both ends to a polypeptide chain.
Where immunoglobulins are used in in vivo diagnosis or therapy, antisera from an allogenic host or from a monoclonal antibody may be immunogenic. Furthermore, when conjugates of other molecules to the antibody are employed, the resulting conjugate may become immunogenic and elicit host antibodies against the constant region of the immunoglobulin or against any other part of the molecule.
It is therefore important that methods be developed which permit the preparation of homogeneous compositions having high specificity for a particular ligand, while avoiding the shortcomings of complete immunoglobulins, and providing the many advantages of lower molecular weight.
2. Description of the Prior Art
Discussions concerning variable regions of heavy and light chains of immunoglobulins may be found in Sharon and Givol, Biochem. (1976) 15:1591-1594; Rosenblatt and Haber, Biochem. (1978) 17:3877-3882; and Early and Hood, Genetic Engineering (1981) 3:157-188. Synthesis of part of a mouse immunoglobulin light chain in a bacterial clone is described by Amster et al., Nucleic Acids Res. (1980) 8:2055-2065. See also the references cited throughout the specification concerning particular methodologies and compositions.